Magazines and similar publications are manufactured by assembling individual page sets, known as signatures, into a group of signatures known as a book. This assembly takes place on a chain conveyor having a number of pockets that move in turn past signature feeders, each containing a different signature for the book being assembled. Signatures are fed to each chain station from some or all signature feeders, depending on the composition of a particular magazine. The resulting book, or group of signatures, then is trimmed and stapled to complete the magazine. Individual signature feeders may be selectively actuated as each chain station passes that feeder, so as to customize the contents of each assembled magazine according to subscriber information or other data known to the printer. The foregoing and related details of magazine production are known to those skilled in the art.
It is important that each assembled book contain the proper number and selection of signatures. If a particular magazine contains more than the intended number of signatures, that magazine wastes the resources of the printer or publisher and also may appear defective to readers. If a magazine contains fewer signatures than desired, readers likely will detect the missing pages and any advertisers on those pages are entitled to compensation from the printer or publisher of the defective magazine. In either case, printers usually measure the thickness of each book assembled pocket of the chain conveyor, to determine whether that book contains the appropriate number of pages. Books that are thicker or thinner than appropriate are diverted from the remaining steps of the assembly operation.
Books of signature groups usually are calipered while moving along the chain by passing each book between a pair of rollers. The rollers compress the pages making up one side of the book, so that the spacing between the rollers is determined by the thickness of the book, that is, by the number of signatures in that book.
Such books are known to have an amount of resilience or elasticity, depending on variable factors such as the number of signatures in the group, the weight of the various signatures making up that group, atmospheric conditions (which may change during a printing run) in the printing plant, and other factors. This resiliency causes the caliper rollers to undergo a degree of bounce relative to each other, as the leading edge of each book enters the nip between the rollers. To avoid an erroneous thickness indication by the caliper, it has been customary to delay measuring the spacing between caliper rollers for a time sufficient to allow this resilient bounce to settle out. Because the amount of dwell time between the caliper rollers for each book is inversely related to the linear travel speed of the chain conveyor carrying the books, the maximum becomes limited as the dwell time diminishes to the minimum time required for the moveable caliper roller to settle out from the bounce induced by the arrival of each book and the elasticity inherent in that book. This limitation on the chain speed limits the throughput, and thus the efficiency, of manufacturing magazines or the like.